Nowadays several image acquisition systems exist that render medical images such as mammographic images in the form of digital signal representations.
An example of such an image acquisition system is a system based on temporary storage of the medical image such as a mammographic image on a photo-stimulable phosphor screen.
A digital signal representation of the medical image is obtained by scanning a screen which has been exposed to a radiation image by means of stimulating radiation, detecting light emitted by the screen upon stimulation and converting the detected light into an electric signal representation.
Another example of such an image acquisition system is a so-called direct radiography system wherein a radiation image is recorded by a two-dimensional array of radiation sensitive elements that, upon addressing, render a digital signal representation of the radiation image.
Still other examples may be envisaged.
For the purpose of diagnosis commonly a visible reproduction, either in the form of a hard copy or a soft copy is generated from the digital medical image.
To that end the digital signal representation of the medical image is applied to a hard copy recorder such as a thermal printer.
In case the diagnosis is performed on the hard copy image, the hard copy is attached to a viewing box for visual inspection and examination by the radiologist.
In order to obtain constant image quality of the hard copy recorded by a hard copy recorder, a regular evaluation of the image quality of the device is to be performed.
The relevant regulation such as the ‘Mammography Quality Standard Act (MQSA)’ of the Food and Drug Administration (FDA) prescribes the contents and the frequency of the measurements to be performed. For mammographic image recording a number of reference values need to be tested before initial use of the recorder and, once these test are satisfactory, quality tests must be performed regularly. Some measurements are to be made on a daily basis, others on a weekly and on a semi-annual basis.
The results of the tests must be saved in order to comply inter alia with the above-mentioned MQSA.
The prior art test method generally consists of three major steps: printing a test pattern, measuring certain values on the printed test pattern, evaluating the measured values relative to reference values.
Commonly after installation of the recorder and before initial use, a test procedure is performed to generate base line values for further follow up. These values must also be determined again after total overhaul and major repair.
The daily measurements comprise: measurement of fog density, mid density, maximum density and density difference between two predefined values.
The weekly evaluations comprise a visual test under controlled conditions following a prescribed procedure. According to this test the visibility of a number of white dots situated in a black background is tested.
The monthly test comprises geometric measurements, i.e. length of edges of a printed rectangular need to be measured.
It is also possible to calculate certain values from measured values.
Next, according to the prior art method, the measured and/or calculated values are copied onto so-called quality control charts. In addition to the measured values the following data regarding the test conditions are also to be filled out:                type and serial number of the mammographic system, type and serial number of the recorder, target maximum density during calibration, type and emulsion number of the recording material, type and serial number of external densitometer.        
Finally the measured values are compared with preset limits. If the measured values do not reach a preset minimum limit or exceed a preset maximum limit, suitable measures are taken. The action must be documented.
The above-described daily test procedure takes a lot of time, it is estimated that about 30 minutes per day are spent in performing the daily test procedure.
Furthermore, the test procedure requires the intervention of a trained operator who is acquainted with the procedure to be performed.
The operator needs to be present at the location of the recorder.
He needs to perform a number of manual operations.
Such a method is time-consuming and error prone.
It is an aspect of the present invention to provide a quality assessment method and system that overcomes the prior art disadvantages.